The invention relates in general to a process for cutting logs to produce thin sheets of wood and, more particularly, to a process for obtaining thin sheets of wood, used for decorative purposes, sliced according to a new and useful technique, which offers the possibility of obtaining, in relation to the different kinds and forms of timbers, the maximum qualitative and quantitative yield of a slicing operation.
It is well known that the demand for thin sheets of wood called "veneers" is continually increasing and is exceeding supply possibilities of actual forests which are offering logs smaller and smaller in size, therefore presenting more difficulties for their utilization in the veneer slicing industry (for example, oak and ash). Consequently, the essential problems connected with the production of such sliced veneers are becoming ever increasingly urgent and vital, and these problems can be summarized in the following paragraphs:
(a) maximum quantitative utilization of a log of wood, by using as much as possible of it for obtaining thin and good quality veneers, thereby minimizing waste and the parts which give poor quality veneers;
(b) for the same number of veneers of a given thickness, obtained from a given log, the obtaining of the greatest possible width with no detriment to the decorative quality;
(c) obtaining of the maximum number possible of veneers with optimum aspect, that is, with regular design, and with grain uniformly distributed at the two sides of the center line of the veneer and not excessively close or open.
Until now, it has not been possible with known means either to obtain the above three results simultaneously or has it been possible to obtain the third of them (subsection (c)) save only for a small part of the cut log. Until now, moreover, in order to obtain sliced sheets of wood for decorative purposes, two essentially different processes have been used based respectively on the two basic types of machines already known and used in this field.
The first, flat slicing process, uses veneer slicing machines in which the log and cutting tools are provided with straight-line reciprocating motion in a direction which can be, in accordance with the types, horizontal, vertical or inclined at any angle. With this process, flat veneers of constant and uniform thickness are obtained.
The record is an eccentric peeling process that uses a rotary veneer lathe in which the log turns about a fixed axis, eccentrically with respect to this axis, while the cutting tools are traversed radially towards the fixed axis of log rotation. With this process, curved veneers are obtained instead with constant and uniform thickness and with relatively marked curvature.
With the flat slicing process, good results are generally obtained for a small part of the log as regards the aspect of the veneers (see point (c)), but the results are modest with respect to the quantitative yield (see points (a) and (b)). In fact, with small size logs, "quartering" is not carried out because it would not be economical due to the heavy incidence on cutting times and the waste in cutting up and squaring up the blocks and for the trimming of the veneers. On the other hand, when the half log or indeed the entire log, is sliced, difficulties almost always occur when cutting close to the center or heartwood of the log, with poor results as regards the standard of finish and surface integrity of the veneer obtained. Consequently, the quantitative yield can drop down to about 50 percent and less; furthermore, veneers are obtained with average widths a good deal less than the log diameter, owing to the practical impossibility of slicing the widest part of the log in the vicinity of the center. This practical impossibility is derived from the physical constitution of many qualities of timber, oak being typical of these, which is, among other things, becoming increasingly more prized in the sliced veneer field. In these qualities of timber, the growth rings of the log are clearly marked and differ into two concentric parts, that of the spring growth being somewhat tender and yielding, and that of the summer growth being, in contrast, hard and fibrous; and, sometimes with the addition, as in the typical case of oak, of "medullary rays", that is, hard and compact fibres radiating from the centre of the log towards the edge. These characteristics bring about, as can be seen in more detail later on, difficulties in cutting which become greater and greater the larger is the angle .alpha. formed by the cutting direction with the tangent of the grain at the point of contact with the grain, more especially in the vicinity of the central zone of the log.
In the eccentric peeling process, which uses a rotary veneer lathe, optimum results are, instead, obtained as regards the quantitative yield (points (a) and (b)), which are, however, accompanied by poor results with respect to the surface finish (point (c)). In fact, the high curvature of cut of the veneers offers the advantage of appreciable increase of the average veneer width and the considerable reduction of angle .alpha. mentioned above. Furthermore, since the angle of intersection of the cutting paths increase with the medullar rays, it is possible to extend the slicing operation over the entire half log right up to the centre. On the other hand, however, as the cutting paths are almost tangential to the growth rings, excessive opening of the grain pattern occurs which appears on the surface of the sliced veneer.